In recent years, regions for development of crude oil, natural gas, and other energy resources have been shifting to the North Sea, Siberia, Northern America, Sakhalin, and other frigid areas and further to the North Sea, Gulf of Mexico, Black Sea, Mediterranean, Indian Ocean, and other deep seas, that is, regions of harsh natural environments. Further, from the viewpoint of the emphasis on prevention of global warming, there has been an increase in development of natural gas. At the same time, from the economical viewpoint of pipeline systems, reduction of the weight of the steel materials and increase in the operating pressure has been sought. The properties sought from line-pipes have become increasingly sophisticated and diverse in accordance with these changes in environmental conditions. They may be roughly classified into demands for (1) greater wall thickness/higher strength, (2) higher toughness, (3) reduction of the carbon equivalent (Ceq) accompanying improvement of on-site weldability (circumferential direction weldability), (4) increased corrosion resistance, and (5) high deformation performance in frozen ground and earthquake/fault line belts. Further, these properties are usually demanded in combination along with the usage environments.
Furthermore, with the backdrop of the recent increase in crude oil and natural gas demand, far off locations and regions of tough natural environments which have been passed over for development due to their unprofitability are starting to be exploited in earnest. In particular, the line-pipes used for pipelines transporting crude oil and natural gas over long distances are being strongly required to be increased in thickness and strength for improving the transport efficiency and also to be increased in toughness so as to be able to withstand use in frigid areas. Achievement of both of these demanded properties is becoming a pressing technical issue.
On the other hand, steel pipe for line-pipes can be classified by its process of production into seamless steel pipe, UOE steel pipe, seam welded steel pipe, and spiral steel pipe. These are selected according to the application, size, etc., but with the exception of seamless steel pipe, each by nature is made by shaping steel plate or steel strip into a tubular form, then welding the seam to obtain a steel pipe product.
Furthermore, these welded steel pipes can be classified according to if they use hot coil or use plate for the materials. The former are seam welded steel pipe and spiral steel pipe, while the latter are UOE steel pipe. For high strength, large diameter, thick wall applications, the latter UOE steel pipe is generally used, but for cost and speed of delivery, the former seam welded steel pipe and spiral steel pipe made using hot coil as a material are being required to be made higher in strength, larger in diameter, and thicker in walls:
In UOE steel pipe, technology for production of high strength steel pipe corresponding to the X120 grade has been disclosed (for example, see “Nippon Steel Monthly”, No. 380, 2004, page 70).
However, the above art is predicated on use of thick-gauge plate as a material. To achieve both higher strength and greater wall thickness, a feature of the thick-gauge plate production process, that is, interrupted direct quench (IDQ), is used at a high cooling rate and low cooling stop temperature. In particular, to secure strength, quench strengthening (texture strengthening) is being used.
As opposed to this, with the hot coil material of seam welded steel pipe and spiral steel pipe covered by the present invention, there is the feature of the coiling process. Due to restrictions in the capacity of coilers, it is difficult to coil a thick-gauge material at a low temperature, so it is impossible to stop the cooling at the low temperature required for quench strengthening. Therefore, securing strength by quench strengthening is difficult.
On the other hand, as technology for achieving both the higher strength and greater wall thickness and the low temperature toughness of hot coil for line-pipes, the technology has been disclosed of adding Ca—Si at the time of refining to make the inclusions spherical, adding V with the crystal refinement effect in addition to the strengthening elements of Nb, Ti, Mo, and Ni, and, furthermore, making the microstructure bainitic ferrite or acicular ferrite to secure the strength by combining low temperature rolling and low temperature cooling (for example, see Japanese Patent No. 3846729 (Japanese Patent Publication (A) No. 2005-503483)).
However, to avoid crack starting points occurring due to brittle fracture from ending up propagating endlessly due to unstable ductile fracture, sought not in petroleum but particularly gas line-pipes, it is necessary to increase the absorption energy at the pipe line usage temperature, but the above art not only does not allude to the art of suppressing the drop in absorption energy due to the occurrence of separation (art of improvement of unstable ductile fracture resistance), but also requires the addition of a certain amount or more of the extremely expensive alloy element V among the alloy elements. This not only invites an increase in cost, but also is liable to reduce the on-site weldability.
Further, from the viewpoint of lowering the transition temperature, art taking note of separation and actively utilizing it is disclosed (for example, see Japanese Patent Publication (A) No. 8-85841). However, the increase in separation improves the low temperature toughness, but on the other hand ends up reducing the absorption energy, so there is the problem that the unstable ductile fracture resistance is caused to deteriorate.